Phase optimization technique in high-speed simultaneous bidirectional links

ABSTRACT

A bidirectional transceiver includes a transmitter and a receiver that respectively transmits a local signal to and receives remote signal from a common bidirectional communication channel, thus the bidirectional channel signal is the superimposition of the local and remote signals. The bidirectional transceiver also includes a transmit canceller that substantially removes the local transmitted signal from the superimposed signals on the bidirectional channel before the local receiver. The remote signal is transmitted by a remote transceiver over the bidirectional channel. A sampling phase is set, based on timing information in the received remote signal, and the received signal is sampled. Timing relation of transitions in the local transmit signal relative to the receiver sampling phase is set such that transmit signal cancellation is optimum at receiver sampling phase, by changing the delay applied to the local transmit signal. To keep the timing relation of the local transmit signal relative to the remote transceiver, a second delay is applied to the local transmit signal before transmission into the bidirectional channel that provides a delay substantially same as the first delay but opposite in direction.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation in part of U.S. patent applicationSer. No. 15/286,538, filed Oct. 5, 2015 titled “Phase OptimizationTechnique in High-Speed Simultaneous Bidirectional Links”, which claimsbenefit priority to U.S. Provisional Patent Application Ser. No.62/238,109, filed on Oct. 6, 2015 titled “A Receive Sampling PhaseOptimization Technique in Multi-Gbps Bi-Directional Serial Links”, theentire contents of each such application are incorporated by referenceherein.

TECHNICAL FIELD

The present disclosure relates generally to the field of communication,and specifically to high-speed simultaneous bi-directional serial links

BACKGROUND

One technique for simultaneous bi-directional data transmission isseparate communication channels. This can be inefficient, though, asavailable channel capacity in the opposite direction can be unused. Suchinefficiency can be costly, particularly where bandwidth is limited(e.g., wireless communication), the transmission distance is long (e.g.,long-haul fiber optic communications), or the number of availablechannels is limited or costly (e.g., high-throughput integrated circuitpackages and printed circuit boards (PCBs).

Another technique for simultaneous bi-directional communication is toassign different carrier frequencies for signals traveling in oppositedirections. The near-side transmit signal can be modulated by onecarrier frequency, and the received signal from the far-side ismodulated by another carrier frequency. The difference between thecarrier frequencies can be enough so the transmit and receive spectrumsdo not overlap and can be properly filtered out at each receiver. Thismethod can be inefficient because it splits the effective availablechannel bandwidth between the two directions, which can bedisadvantageous in high-throughput applications. Additionally, therequired frequency separation margin, for proper channel isolation andfiltering of signals in each band, can lead to wasteful use of thechannel bandwidth. For example, operation of a simultaneousbi-directional communication using separate frequency channels in eachdirection can waste half or more of the total available channelbandwidth.

Another technique for simultaneous bi-directional transmission operatestransceivers at opposite ends of a channel medium, concurrentlytransmitting signals to one another other, through the channel medium,such that the receiver of each transceiver receives a superposition ofthe signal sent by the opposite end transceiver, and the signaltransmitted by its own transmitter. One technique for removing thesignal transmitted by its own transmitter is to generate, locally, areplica of that transmitted signal and then subtract the replica fromthe superposition, then sample the residual. If the replica is exact,the only signal remaining in the residual is the signal from theopposite end transceiver. However, there can be problems with thistechnique, particularly at higher communication bandwidths. Certain ofthe problems can arise from fast slew rate transitions of thetransceiver's own transmitted signal, as these can introduce noise,especially if proximal in time to a trigger or sampling phase of thereceiver's sampler. However, conventional techniques cannot simply shiftthe sampling phase to move it away from transitions in the locallytransmitted signal, because the sampling phase is optimized in relationto the symbol period in the received signal. Merely shifting thesampling phase could result in sub-optimal sampling, which in turn couldproduce unacceptably high error rates.

There is a need therefore for a practical, stable performancesimultaneous bi-directional transceiver system in which opposite endtransceivers can each cancel receipt of their own transmissions, whilemaintaining in the each of the transceivers an optimal sampling of thesignal received from the other.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 Illustrates one example bidirectional communication link over asingle physical channel.

FIG. 2A Illustrates a transmitter eye diagram for a non-return to zero(NRZ) data pattern before the physical channel.

FIG. 2B Illustrates an attenuated received eye diagram after a low-passchannel, with the strong local transmit signal transitioning at middleof received eye opening.

FIG. 2C Illustrates an attenuated received eye diagram after a low-passchannel, with local transmit transition timing shifted away from thereceived eye opening.

FIG. 3A Illustrates an example implementation of a phase optimizingsimultaneous bidirectional link according to various aspects.

FIG. 3B Illustrates another example implementation of a phase optimizingsimultaneous bidirectional link according to various aspects.

FIG. 4A Illustrates an example alternative implementation of a phaseoptimizing simultaneous bidirectional link according to various aspects.

FIG. 4B Illustrates another example alternative implementation of aphase optimizing simultaneous bidirectional link according to variousaspects.

FIG. 5 Illustrates an example of another alternative implementation of aphase optimizing simultaneous bidirectional link according to variousaspects.

FIG. 6 Illustrates an example of another alternative implementation of aphase optimizing simultaneous bidirectional link according to variousaspects.

FIG. 7 is a flow diagram of operations to adjust delay elements in animplementation of a phase optimizing simultaneous bidirectional linkaccording to various aspects, to optimize sampling points in accordancewith disclosed concepts and aspects thereof.

DETAILED DESCRIPTION

FIG. 1 illustrates an example simultaneous bi-directional transceiversystem capable of providing recovery, at each of the bi-directionaltransceivers 101 and 102, of the signal transmitted through the physicalchannel 103 by the other. Transceiver 101 includes transmitter 104,receiver 105, transmit replica unit 106, signal adder 107, and a clockand data recovery (CDR) unit 108. Transceiver 102 is similarlyconfigured, and therefore labeling is omitted. Operation will bedescribed for the transceiver 101, and will be understood to apply inlike manner to transceiver 102. In operation, the line signal at thebi-directional transceiver 101 is a superposition of the transmit signaloutput by its own transmitter 104 and a signal received, aftertravelling through the channel 103, from the transmitter of transceiver102. The transceiver 101 removes its own transmit signal from thesuperposition by configuring the transmit replica unit 106 to generatesubstantially the same signal as transmitter 104 but with oppositepolarity, then using signal adder 107 to add the opposite polarityreplica signal and the channel signal and outputs the sum as a residualto the receiver 105. Operations of the CDR unit 108 will be described ingreater detail in reference to FIGS. 2A and 2B. Referring to FIG. 1,assuming the transmit replica unit 106 generates an accurate replica(with opposite polarity), the residual will include none of the signaltransmitted by transmitter 104. However, as the communication signalbandwidth increases, certain phenomena can arise, including significantattenuation by the physical channel 103 of the signal from the oppositeend transceiver, that can complicate, or degrade, or both, theabove-described transmit signal cancellation.

Referring to FIGS. 2A and 2B, information represented by the drawingswill be described referring to FIG. 1, and will assign transceiver 101as a “local” transceiver and transceiver 102 as a “remote” transceiver.Referring to FIG. 2A, illustrated is a transmit signal eye diagram atthe remote transceiver (e.g., at transceiver 102) before the channel103. FIG. 2B shows the attenuated received signal, as received at thelocal transceiver (e.g., transceiver 101) after passing through thechannel 103. At the local transceiver, the CDR unit 108 determinesoptimum sampling phase. A conventional CDR unit extracts the phase andfrequency information from the received signal and generates a localclock that is frequency locked to the received signal with a samplingphase for highest received SNR. In a high-speed simultaneousbidirectional link, the received signal eye opening is significantlysmaller compared to the local transmit amplitude on the same channel.Thus, small residual errors in cancellation of the relatively largetransmit signal leads to large degradations of the received signal.Large transmit cancellation errors specifically occur during the fastsignal transitions, as any minor timing error such as phase error orjitter translates to large amplitude errors. FIG. 2B shows an example ofa highly attenuated received signal with a large local transmit signaltransition at middle of the received eye opening. This conditionrepresents a worst case scenario for this problem, because transmitmaximum slew rate occurs at the same phase location as CDR unit choosesas optimum receive sampling point. Therefore a conventional CDRfunctionality cannot provide an optimum sampling condition for receiveSNR.

Referring to FIG. 1 simultaneous bidirectional link 100, there are twosignals whose phase alignments are important, the conventional CDR andconventional circuit combinations arrangement circuit provide only onedegree of freedom in phase selection. As will be appreciated andunderstood from reading this disclosure, implementations and aspects ofdisclosed methods and systems can provide two degrees of freedom. One isthe control of timing between sampling clock phase and received signal.The second is shifting the transmit transitions of the local transmittedsignal away from the received signal optimum sampling time selected bythe CDR. This condition is shown in FIG. 2C, where the earlier transmittransitions (occurring at middle of the receive eye) are shifted by halfa symbol time to have the maximum distance from the received center eye.In an aspect, the additional timing control can be configured to shiftthe fastest transmit slew rate region (around transmit zero crossings)in between two consecutive optimum receive sampling points. Suchrelative phase adjustment, as shown in FIG. 2C, can align the lowestslew rate portion of the transmit signal with the receiver optimumsampling point, thus minimizes the transmit cancellation error.

FIG. 3A shows an implementation of an example simultaneous bidirectionallink 300 according to various aspects. Referring to FIG. 3A a delay unit302, which will be referred to as a “relative delay” unit 302 isarranged in a first bidirectional transceiver 304 that, when controlledas will be described, sets the relative delay between the local transmittransitions and the optimal receiver sampling phase. By providing thisnew degree of freedom between the local transmit and receive timing, therelative delay unit 302 can shift local transmit transitions away fromoptimal receiver sampling phase as determined by a timing or clockrecovery unit 306. Referring to FIG. 3A, there can be a second or remotebidirectional transceiver 308 on the other side of the bidirectionalchannel 310. In an implementation, as illustrated in FIG. 3A, theconfiguration and components of the remote bidirectional transceiver 308may be identical to or substantially the same as those of the firstbidirectional transceiver.

Accordingly, to avoid obscuring relevant details in FIG. 3A, explicitlabeling and numbering of items in the remote bidirectional transceiver308 is omitted. In some embodiments, each of the items is implicitlynumbered and labeled identical to the corresponding item of the firstbidirectional transceiver 304.

Another application of the bidirectional delay line 312 is to provide ameans for removing the differential skew in the positive and negativepolarities of the received signal. As a differential signal travelsthrough a channel, the positive and negative polarities of the signalexperience different delays as they travel through separate paths. Suchdelay difference is not noticeable at lower data rates, because it is asmall percentage of the each data symbol. However, such delay differencebecomes a larger portion of a data symbol at very high data rates andwill affect signal quality at the receiver severely. This delaydifference can be removed in the same transceiver architectures using abidirectional delay element as shown in FIGS. 3A to 6, for example byapplying different delay adjustment controls to the positive andnegative paths in the bidirectional delay element.

To make this concept easier to follow, FIGS. 3B and 4B are shown indifferential mode. For example, if one assumes the two positive andnegative polarities of the far-end signals are received with a skewdelay between them at transceiver 304, where positive signal is ahead ofthe negative signal by a timing skew of Xt. The bidirectional delayelement 312 can be adjusted such that its positive delay path delays thepositive signal by Xt more than it delay its negative delay path, suchthat at the point the two polarities arrive at the summer 320, the skewbetween the two polarities are substantially cancelled. To avoid thisdelay skew between the positive and negative paths of bidirectionaldelay line 312 affect the transmit signal going out, the positive andnegative transmit signals should be skewed in opposite direction beforebidirectional delay element 312. This opposite differential skew can beapplied by applying different delay skews for positive and negative pathof delay element 302.

In many applications that transmit driver is inherently differential andone cannot apply the differential delay to the output of the driver, adifferential delay element can be added between the transmit driver 316and the summer 320, as shown in FIG. 4B.

The same or similar mechanism as described above in reference to therelative delay unit 302 can exist in a second, or remote bidirectionaltransceiver 308 on the other side of the bidirectional channel 310 inorder to address the same timing adjustment between its own transmitsignal and the received signal from the first bidirectional transceiver304. However, the transmit timing adjustment in the first bidirectionaltransceiver 304 on one side of the bidirectional channel 310 leads to atiming change for the received signal by the remote bidirectionaltransceiver 308 at the other side of the bidirectional channel 310. As aresult, the remote bidirectional transceiver 308 needs to shift itsoptimal receiver sampling phase to track the timing change of the signalreceived from the bidirectional channel, and additionally shift itstransmit timing together with its receiver sampling phase to keep theirrelative timing the same, such that the transmit transitions at receivercontinue to occur in between its receive sampling points. This transmittiming change by the remote bidirectional transceiver 308 leads to thesame timing shift in the signal received by first bidirectionaltransceiver 304, thus negates the original optimum timing adjustmentbetween transmit and received signal, which was set by relative delayunit 302. To address this problem, a timing shift (Delta), opposite inpolarity but substantially equal in magnitude to that of the relativedelay unit 302, can be added to the bidirectional channel 310. This canbe accomplished by arranging a bidirectional delay unit 312 in the firstbidirectional transceiver 304, as shown in FIGS. 3A and 3B. Togetherwith the relative delay unit 302, the bidirectional delay unit 312 canprovide the required relative delay adjustment between local transmitand received signal, while keeping the net delay change of thetransmitted signal into bidirectional channel substantially the same. Inthis scheme, the effective relative delay change between the transmitand receive timing is the sum of the delay magnitude changes (Delta)applied by the relative delay unit 302 and the new bidirectional delayunit.

Since in a normal operation, the delay change (Delta) in the two delayunits (relative delay unit 302 and bidirectional delay unit 312) need tohave substantially the same magnitude, the effective relative delaychange between transmit and received signals at receiver sampling pointis the sum of the delay change of each of the delay units (i.e.,2×Delta). While one bi-directional delay unit is sufficient to addressthe timing conflict between the transceivers on two sides of thebi-directional channel, the remote bidirectional transceiver 308 caninclude a similar bi-directional delay unit 312 to provide wider delayadjustment for the link.

Referring to FIGS. 3A and 3B, the relative delay unit 302 is shownarranged preceding the transmitter 316 and transmit replica unit 318.This arrangement can provide alignment of the output of the transmitter316 to the output of the transmitter replica unit 318 feeding thecanceller unit 310. However, as will be shown in reference to FIGS. 4Aand 4B, the relative delay unit can serve the same function if it isreplaced by one duplicate placed after the transmitter 316 and anotherduplicate after transmit replica unit 318, as long as it does not affectthe receive signal.

FIGS. 4A and 4B show examples where the relative delay is applied aftertransmitter 316 and transmit replica unit 318, by use of a firstrelative delay unit 402 at the output of the transmitter 316, and asecond relative delay unit 404 after the transmit replica unit 318, butbefore the canceller unit 320 in the path to the receiver 322.

Another permutation of such architecture is shown in FIG. 5, where therelative delay is still applied after the transmitter 316 by the firstrelative delay unit 402, but is applied preceding the transmit replicaunit 318 using relative delay unit 502.

Another permutation of the same architecture is shown in FIG. 6, wherethe relative delay is applied preceding the transmitter 316 by the firstrelative delay unit 602, but is applied after the transmit replica unit318 using relative delay unit 604.

Referring again to FIGS. 3A and 3B, for purposes of description, achannel coupling the output of the transmitter 316 to one end of thebidirectional delay unit 302 and to an input of the canceller unit 320can be termed a “local signal channel” (visible, but not separatelynumbered in the drawings). Also for purposes of description, thetransmit replica unit 318 and the canceller unit 320 can be collectivelyreferenced as a “cancellation circuit” (visible, but not separatelynumbered in the drawings). The cancellation circuit, as can beunderstood from description herein of operation of its components, canbe configured to generate a replica of the delayed driver signal, (e.g.,the output of the transmitter 316 in FIGS. 3A and 3B, for example) andto receive a signal from the local signal channel, and furtherconfigured to generate a receiver input signal (to the receiver 322)based on subtracting the replica of the delayed driver signal (from thetransmit replica unit 318) from the signal received from the localsignal channel.

FIG. 7 illustrates an example flow 700 of operations in a method oftiming adjustment between local transmitter transition and receiversampling for best transmit signal cancellation at receive optimalsampling phase in a simultaneous bidirectional link, using system ofFIG. 3A or 3B. Persons of ordinary skill, upon reading this disclosure,can readily apply the flow 700 to equivalent operations using eitherFIG. 4A, FIG. 4B, FIG. 5, or FIG. 6 system. Referring to FIGS. 3A and3B, at step 702 transceiver 304 starts transmission of a local signalinto the bi-directional channel 310, and the cancellation circuitoperates to remove that local transmit signal before being received bythe receiver. At steps 704 and 706, transceiver 304 receives the signalfrom the channel 310 and receiver 322, using CDR unit 306, recovers thebest phase to sample the received signal. Once the best sampling pointwith regards to the received signal is identified, transceiver 304internally determines if the residual local transmit signal has aminimal slope at the receiver sampling phase. If the answer is “yes,” noaction is taken and transceiver continues to track the receive signalphase. Otherwise, at step 710, transceiver 304 uses relative delay unit302 and bidirectional delay unit 312 to shift the timing of the transmittransitions away from the sampling phase at the receiver 322, butwithout shifting the phase of the transmitted signal into channel 310.The relative timing between transmit and receive is selected, at step712, such that the residual local transmit signal has a minimal slope atthe receive sampling phase. As a result, after the adder or cancellerunit 320, receiver sampling occurs at a most stable portion of the localtransmit signal with minimal slope, or effectively at the middle of thetransmit eye. As discussed earlier, at this point the transmit signalcancellation can be performed most accurately and optimally.

It can be noted by persons of ordinary skill, upon reading thisdisclosure that, for a known channel with a constant delay, the optimumrelative delay setting between transmit and receive timing of abidirectional transceiver can be identified a priori and hard-coded inthe relative delay unit and the bidirectional delay unit in advance.Additionally, in certain implementations that the delay of thebi-directional channel itself can be controlled or its delay can beproperly set in advance by design for a target signaling scheme, thebidirectional delay unit can be eliminated from the transceiver.

While this invention has been particularly shown and described withreferences to preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade therein without departing from the scope of the inventionencompassed by the appended claims.

What is claimed is:
 1. A bi-directional transceiver circuit comprising:a differential delay driver circuit, configured to receive a data-insignal and to drive into a differential local signal channel adifferential delayed driver signal representing the data-in signal,having positive and negative polarity, and a first time shift for itspositive polarity and a second time shift for its negative polarity; abi-directional differential delay element having positive and negativepolarities and different time shifts for its positive and negativepolarities, configured to couple the local signal channel with anexternal channel medium; a cancellation circuit, configured to generatea replica of the differential delayed driver signal, and to receivesignals from the local signal channel, and further configured togenerate a receiver input signal based on subtracting the replica of thedifferential delayed driver signal from the signals received from thelocal signal channel; the bi-directional differential delay elementfurther configured to receive from the external channel medium adifferential signal with timing skew between its positive and negativepolarities.
 2. The bi-directional transceiver circuit of claim 1,wherein the differential delay driver circuit is a settable differentialdelay driver circuit, further configured to receive first and seconddelay settings, and wherein the first time shift is adjusted inaccordance with the first delay setting and the second time shift isadjusted in accordance with the second delay setting.
 3. Thebi-directional transceiver circuit of claim 2, wherein the delaysettings are delay control signals, and wherein the bi-directionaltransceiver circuit further comprises: a local transmit phasecontroller, configured to generate the delay control signals based on atiming of transitions in the differential delayed driver signal relativeto a sampling phase.
 4. The bi-directional transceiver circuit of claim3, wherein the local transmit phase controller is further configured toadjust the delay control signals to maintain a given optimal phasedifference between the timing of transitions in the differential delayeddriver signal and the sampling phase.
 5. The bi-directional transceivercircuit of claim 4, wherein the given optimal phase difference is wherethe transmit signal is maximally canceled at the sampling phase.
 6. Thebi-directional transceiver circuit of claim 3, wherein: the settabledifferential delay driver circuit is configured to change the first timeshift by an first Delta, the first Delta being a time shiftcorresponding to the first delay control signal, and the second timeshift by a second Delta, the second Delta being a time shiftcorresponding to the second delay control signal, and the bi-directionaldifferential delay element, configured to provide a bi-directionaldifferential path, having a third time shift between positive polarityof the external channel medium and the positive polarity of local signalchannel, and configured to change the third time shift by a third Delta,the third Delta having the same or substantially the same magnitude as amagnitude of the first Delta and a polarity opposite to a polarity ofthe first Delta, and the bi-directional differential delay elementfurther having a fourth time shift between negative polarity of theexternal channel medium and the negative polarity of the local signalchannel, and configured to change the fourth time shift by a fourthDelta, the fourth Delta having the same or substantially the samemagnitude as a magnitude of the second Delta and a polarity opposite toa polarity of the second Delta.
 7. The bi-directional transceivercircuit of claim 6, wherein: the settable differential delay drivercircuit includes an input and a differential output, and includes afirst adjustable differential delay element, the first adjustabledifferential delay element configured to provide a differential path,from the driver input, having the first and second time shift, and toreceive the first and second delay control signals, and in response,change the first time shift of the driver output positive polarity bythe first Delta and the second time shift of the driver output negativepolarity by the second Delta, and the cancellation circuit includes adifferential delay replica driver and a differential adder, thedifferential delay replica driver having an input coupled to thedifferential delay driver input, and a differential output, and includesa second adjustable differential delay element, configured to provide adifferential path from the driver input, having a fifth time shift forthe positive polarity of the replica driver output and a sixth timeshift for the negative polarity of the replica driver output, and toreceive the first and second delay control signals, and in response,change the fifth time shift by an amount equal to the first Delta andchange the sixth time shift by an amount substantially equal to thesecond Delta, and the adder having a first adder input configured toreceive signals from the local signal channel, a second adder inputconfigured to receive an output signal from the differential delayreplica driver, and an adder output configured to generate the receiverinput signal based on subtracting the output signal of the differentialdelay replica driver from the received signals of the local signalchannel.
 8. The bi-directional transceiver circuit of claim 7, whereinthe bi-directional adjustable delay element is further configured to:provide the bi-directional path, having the third and fourth time shift,between the positive and negative polarities of an external differentialwireline channel medium and local signal channel, receive the delayeddifferential driver signal from the local signal channel, and transmitinto the external differential wireline channel medium a correspondingdifferential wireline transmit signal, wherein its positive and negativepolarities are delayed by the third and fourth time shifts respectivelyrelative to the positive and negative polarities of the delayeddifferential driver signal, and receive a differential wireline signalfrom the external differential wireline channel medium and deliver, intothe local signal channel in superposition with the delayed differentialdriver signal, a corresponding delayed differential received signal,wherein its positive and negative polarities are delayed by the thirdand fourth time shifts respectively relative to the received wirelesssignal.
 9. The bi-directional transceiver circuit of claim 6, wherein: adifference between the third and fourth time shifts in thebi-directional differential delay element is adjusted so the timing skewbetween the positive and negative polarities of the differential signalreceived from the external channel medium is substantially zero at thelocal signal channel.
 10. A receiver circuit comprising: bi-directionaldifferential delay element with a positive polarity signal path having afirst time shift and a negative polarity signal path with a second timeshift configured to couple a local signal channel to an externaldifferential channel medium; a subtraction circuit, configured toreceive a differential signal from the local signal channel, and furtherconfigured to generate a receiver input signal based on subtractingpolarities of the differential signal received from the local signalchannel.
 11. The receiver circuit of claim 9, wherein: thebi-directional differential delay element is a settable differentialdelay circuit, further configured to receive a first and second delaysettings, and wherein the first time shift is adjusted in accordancewith the first delay setting and the second time shift is adjusted inaccordance with the second delay setting.
 12. The receiver circuit ofclaim 10, wherein: The first and second delay settings are adjusted sothe timing skew between the positive and negative polarities of thedifferential signal received from the external channel medium issubstantially zero at the local signal channel.